In electrostatography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image) is formed on an insulative surface by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise radiation-induced discharge of a uniform potential previously formed on a surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of a pattern of electrostatic potential on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles adhere to the surfaces of the carrier particles. When the developer is brought into contact with the electrostatic latent image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited in the desired amount on the latent image-bearing surface. In order to meet these requirements for proper development, the level of electrostatic charge on the toner and carrier particles should be maintained within an acceptable range. The actual range of charge level that is acceptable or optimum depends upon the nature of the particular toner, carrier, development process, and development apparatus desired to be employed.
Toner particles in dry developers often contain material referred to as a charge agent or charge-control agent, which helps to establish and maintain toner charge within an acceptable range. Many types of charge-control agents have been used and are described in the published patent literature. The level of charge on toner particles can be controlled to some extent by changing either the nature or the amount of the charge agent in the toner particles. However, there are difficulties and limits associated with such approaches. In changing the nature of the charge agent, one must be concerned with the availability of an appropriate charge agent material and with its compatibility, dispersability, and possibility of adverse reaction with the toner or carrier material. In changing the amount of charge agent in toner particles, one often finds that decreasing the amount leads to increased undesirable throw-off of material from the developer during use or that in increasing the amount a limit or plateau is reached in the capability of dispersing the charge agent in the toner particles. Also when the amount of charge agent in the toner is increased above an optimum amount, the charge to mass of the toner decreases. When the amount of charge agent in the toner is decreased below an optimum amount, the charging rate of the toner is decreased.
Therefore, the level of charge that will be created and maintained on the toner is still very dependent on the nature and condition of the carrier particles.
One known method of controlling charge level of a toner involves base or acid washing of the carrier. This technique allows some control of toner charge level but is not very attractive from a manufacturing standpoint and provides little leeway for precise adjustment of charge level.
Many known dry electrostatographic developers contain thermoplastic toner particles and carrier particles that comprise a core material coated with a polymer. Such polymeric carrier coatings can serve a number of known purposes. One such purpose can be to aid the developer to meet the electrostatic force requirements mentioned above by shifting the carrier particles to a position in the triboelectric series different from that of the uncoated carrier core material, in order to adjust the degree of triboelectric charging of both the carrier and toner particles. Another purpose can be to reduce the frictional characteristics of the carrier particles in order to improve developer flow properties. Still another purpose can be to reduce the surface hardness of the carrier particles so that they are less likely to break apart during use and less likely to abrade surfaces (e.g., photoconductive element surfaces) that they contact during use. Yet another purpose can be to reduce the tendency of toner material or other developer additives to become undesirably permanently adhered to carrier surfaces during developer use (often referred to as scumming). A further purpose can be to alter the electrical resistance of the carrier particles.
However, while such carrier coatings can serve the above-noted purposes well, in some cases they do not adequately serve some or all of those purposes simultaneously. For example, depending upon the nature of the toner particles and carrier core material desired to be included in the developer, such carrier coatings can cause the developer to acquire a triboelectric charge that is at an inappropriate level for optimum developer performance.
Some publications describe means for alleviating this problem to some degree by blending polymers or other materials having triboelectric characteristics different from each other and coating the blend on carrier core particles in order to alter the carrier particles' triboelectric charging characteristics more precisely and, in some cases, provide other desirable properties, such as better adhesion of the coating to the core particles. Many different types of polymers have been described as useful for this purpose. See, for example, U.S. Pat. Nos. 4,937,166; 4,725,521; 4,590,440; 4,297,427; and 5,100,754. By altering the ratio of the amounts of materials included in the blend, one can fairly precisely alter the level of triboelectric charge imparted to the carrier particles and to the toner particles with which they are intended to be mixed.
However, such an approach also has drawbacks and limitations. In choosing materials having different triboelectric characteristics to be blended and coated on carrier core particles, one must be concerned with compatibility of the materials. There must not be adverse interaction of the materials with each other that would alter their desired triboelectric charging tendencies. Also, if the materials are not miscible with each other it will not be possible to blend the materials homogeneously, which can result in poor coating adhesion and mechanical integrity and inconsistent triboelectric properties. Furthermore, if the melting temperatures of the materials are significantly different, it will be difficult or impossible to properly coat the blend on core particles by well known melt-coating techniques. Also, because the materials must be blended, only one method of coating the blended materials simultaneously on the cores can be used.
Another drawback inherent in such an approach is that if it is desired to alter the triboelectric charging tendencies of carrier particles coated with a blend, by adjusting the ratio of the amounts of different materials in the blend (e.g., in response to developer aging or to a change in toner material, development process, and/or development apparatus), a new coated carrier must be produced, having an altered ratio of amounts of materials in the coated blend, each time it is desired to effect such a change in charging tendency.
One patent, U.S. Pat. Nos. 3,795,618 discloses the use of a mixture of two different carrier particles to affect the amount of toner charge; however, the toner charge does not either continuously increase (or decrease) with an increase (or decrease) in the weight percent of one of the carriers in the mixture. Therefore, it would not be possible to knowingly increase or decrease the toner charge by adding one type of carrier particles to the mixture without first determining what the exact composition of carriers is in the mixture, or by doing it by trial and error.
Thus, a need still exists for carrier mixtures which avoid the above-noted drawbacks associated with using one type of carrier particle comprising core particles coated with a blend of different materials, i.e., while avoiding the need to be concerned with the compatibility, solubility, miscibility, and matching melting temperatures of materials in blends. Also, a need exists for convenient means for changing the level of toner charge without having to fashion a new type of carrier particle each time it is desired to effect such a change.